1. Field of the Invention
This invention relates to the switching mechanism of a small circuit breaker.
2. Prior Art
FIG. 5 shows essential components of the switching mechanism of a conventional circuit breaker (or molded-case circuit breaker). In FIG. 5, reference numerals 30 designates a latch; 31, a holder; and 32, a toggle link unit. The latch 30 is rotatably supported at one end by means of a latch pin 33; however, normally it is locked by a trip mechanism 34 at the other end so that it may not be turned. The holder 31 holds a movable contactor 35, and is rotatably supported by a switching shaft 36. The toggle link unit comprises an upper link 37 coupled to the latch 30, and a lower link 38 coupled to the holder 31. The upper link 37 is coupled to the lower link 38 with a center pin 39. One end of a switching spring (not shown) is engaged with the center pin 39 so that the toggle link unit 32 is reversed by the tensile force of the switching spring. Further in FIG. 5, reference numeral 40 designates a stationary contactor.
When the latch 30, the holder 31, the toggle link unit 32, and the center pin 39 are positioned as indicated by the solid lines in FIG. 5, the circuit breaker is in the "on" state; and, when the circuit breaker is tripped, they are positioned as indicated by the two-dot chain lines. When, with the circuit breaker in the "on" state, an overcurrent trip device (not shown) operates to disengage the latch 30 from the trip mechanism, the latch 30 is turned counterclockwise by the elastic force of the switching spring (not shown) through the toggle link unit 32. In this operation, whenever the elastic force goes over the dead point the toggle link unit 32 is reversed. As a result of this occurrence the circuit breaker is tripped as indicated by the two-dot chain lines in FIG. 5.
In FIG. 5, reference characters P.sub.1, P.sub.2 and P.sub.3 designate the points where the center pin 39 is positioned during the tripping operation of the circuit breaker. More specifically, P.sub.1 is an "on" position, P.sub.2 is a beyond-dead-point position, and P.sub.3 is a trip position. Further in FIG. 5, reference character M designates the locus of the center pin 39 from the position P.sub.1 to the position P.sub.3. The locus is a curve as shown in FIG. 5, and analysis has proven that it is a combination of curves of different radii.
Improvement of the performance of the circuit breaker resides in reducing the period of time required for opening the circuit breaker as much as possible. For this purpose, the locus of the position P.sub.1 to the position P.sub.3 should be a straight line instead of a curve. That is, if the locus M is a straight line, the center pin 39 goes the shortest distance between the two positions P.sub.1 and P.sub.3 ; that is, the circuit breaker will be opened in the shortest time.